1. Field of the Invention
This invention relates to a crystallized glass suitable for a substrate for an information recording medium such as a magnetic disc, optical disc, or optomagnetic disc, a substrate for information recording medium using such a crystallized glass substrate, and an information recording medium using such a substrate for information recording medium. More particularly, this invention relates to a crystallized glass capable of providing a glass substrate having a high Young's modulus, as well as excellent mechanical strength, surface flatness, and heat resistance and having an excellent surface smoothness upon polishing and to a glass substrate having an excellent surface smoothness using such a crystallized glass.
2. Description of Related Art
Major components of a magnetic recording apparatus in, e.g., a computer, includes a magnetic recording medium and a magnetic head for magnetic recording and reproducing. As a magnetic recording medium, known are a flexible disc and a hard disc. Aluminum alloy, among materials, has been used for the substrate material for hard disc. The floating amount of the magnetic head is significantly reduced in accordance with recent trends that hard disc drives for note type personal computer are made smaller and that the magnetic recording is made with higher density. A very high precision, according to those trends, is required for surface smoothness on the magnetic disc substrate. In the case of an aluminum alloy, however, it is difficult to manufacture a flat surface with a precision of a certain degree or higher, because the polished surface may be plastically deformed due to a low hardness even if a polishing material having a high precision and a machine tool are used for polishing. Even if nickel-phosphorus plating is made on a surface of the aluminum alloy, the surface roughness Ra cannot be controlled at five angstroms or less. According to the developments of the hard disc drives that become smaller and thinner, there are strong demands for making thinner the magnetic disc substrate. The aluminum alloy, however, has low strength and rigidity, and therefore, it is difficult to make the disc thin while the hard disc drive maintains certain strength as required from the specification for the drive.
To solve such problems, a glass substrate for magnetic disc claiming high strength, high rigidity, high impact resistance, and high surface smoothness has been developed. Chemically reinforced glass substrates whose substrate surface is reinforced with an ion exchange method and crystallized substrates subjecting to a crystallization process, inter alia, are known well.
As a glass substrate reinforced by ion exchange, e.g., a glass disclosed in Japanese Unexamined Patent Publication No. 1-239,036 has been known. This ion exchange reinforced glass substrate is made of a glass including, by percent by weight, SiO.sub.2 of 50-65%, Al.sub.2 O.sub.3 of 0.5 to 14%, R.sub.2 O (wherein R denotes alkali metal ion) of 10 to 32%, ZnO of 1 to 15%, and Ba.sub.2 O.sub.3 of 1.1 to 14% where the glass is reinforced by forming compression stress layers on a surface of the glass substrate by an ion exchange method with alkali ions, and the above publication discloses such a glass substrate for magnetic disc.
As a crystallized glass, e.g., there is a disclosure in Japanese Patent Publication No. 2,516,553. This crystallized glass includes, by weight, SiO.sub.2 of 65 to 83%, Li.sub.2 O of 8 to 13%, K.sub.2 O of 0 to 7%, MgO of 0.5 to 5.5%, ZnO of 0 to 5%, PbO of 0 to 5% (provided that MgO+ZnO+PbO is of 0.5 to 5% by weight), P.sub.2 O.sub.5 of 1 to 4%, Al.sub.2 O.sub.3 of 0 to 7%, and As.sub.2 O.sub.3 +Sb.sub.s O.sub.3 of 0 to 2% and is a crystallized glass for magnetic disc including fine crystal particles of LiO.sub.2.cndot.2SiO.sub.2 as a primary crystal.
A crystallized glass is disclosed also in Japanese Unexamined Patent Publication No. 7-291,660. The crystallized glass is obtained by melting a glass composed of, by percent by weight, SiO.sub.2 of 38 to 50%, Al.sub.2 O.sub.3 of 13 to 30%, MgO of 10 to 20% provided that, by weight ratio, Al.sub.2 O.sub.3 /MgO is 1.2 to 2.3, B.sub.2 O.sub.3 of 0 to 5%, CaO of 0 to 5%, BaO of 0 to 5%, SrO of 0 to 5%, ZnO of 0.5 to 7.5%, TiO.sub.2 of 4 to 15%, ZrO.sub.2 of 0 to 5%, and As.sub.s O.sub.3 and/or Sb.sub.2 O.sub.3 of 0 to 2% and thermally treating it after molding, and is a cordierite based crystallized glass having a feature that the glass contains as a primary crystal a cordierite based crystal. The above publication also discloses a substrate for magnetic disc made of a crystallized glass.
Another crystallized glass is also disclosed in Japanese Unexamined Patent Publication No. 9-77,531. This crystallized glass is a glass ceramic product having a Young's modules in a range from about 14.times.10.sup.6 to about 24.times.10.sup.6 psi (96 to 165 Gpa) and a breakdown tenacity more than 1.0 Mpa.cndot.ml/2. The crystallized glass is constituted of a crystal phase conglomerate mainly made of a spinel type crystal uniformly sized and evenly dispersed in the residual glass matrix phase including rich silicon. The glass is substantially made of, by percent by weight, SiO.sub.2 of 35 to 60%, Al.sub.2 O.sub.3 of 20 to 35%, MgO of 0 to 25%, ZnO of 0 to 25%, TiO.sub.2 of 0 to 20%, ZrO.sub.2 of 0 to 10%, Li.sub.2 O of 0 to 2%, NiO of 0 to 8%, wherein the total of MgO and ZnO is at least 10%, and may contain an arbitrary component selected up to 5% from a group constituted of BaO, CaO, PbO, SrO, P.sub.2 O.sub.5, B.sub.2 O.sub.3 and Ga.sub.2 O.sub.3, and R.sub.2 O of 0 to 5% selected from a group constituted of Na.sub.2 O, K.sub.2 O, Rb.sub.s O, and Cs.sub.s O, and a transitional metal oxide of 0 to 8%. The glass may be a glass ceramic having a composition in which the total amount of TiO.sub.2 +ZrO.sub.2 +NiO is of 5% or more in the case where Al.sub.2 O.sub.3 is contained only in an amount of about 25% or less, and the above publication also discloses a substrate for magnetic disc made of a glass ceramic.
However, in accordance with recent trends that the hard discs are made smaller and thinner and that the recording is made with a higher density, flying height of the magnetic head is lower, and the disc is rotated at a higher speed, so that strength, Young's modulus, and surface smoothness of the disc substrate material are further severely needed. Particularly, the surface smoothness and surface flatness of the substrate material are strictly on demands due to trends for higher density information recording on 3.5 inch hard discs for personal computers and servers, and the disc has to be rotated at 10,000 rpm or more in corresponding to higher speed of data processing. While the rigidity of the substrate material is subject to a further strict standard, it is apparent that the conventional aluminum substrate is already limiting itself. As far as demands for disc drives having a higher capacity and higher speed are necessary from now on, a substrate material for magnetic recording medium is required doubtlessly to have high Young's modulus, high strength, excellent surface flatness, good impact resistance, and so on.
The necessity for high Young's modulus can be illustrated based on the following facts. That is, according to recent trends for HDDs (hard disc drives) which are made smaller with high capacity and high operation speed, future substrates for magnetic recording medium may have the thickness of 0.635 mm, currently 0.8 mm in the case of 3.5 inches and of 0.43 mm or 0.38 mm, currently 0.635 mm in the case of 2.5 inches, and the rotation speed of the substrate may predictably be made higher to 140,000 rpm from current 100,000 rpm as the maximum speed. Such a substrate for magnetic recording medium may tend to sustain more looseness and undulation and warp as the substrate for magnetic recording medium becomes thinner, and the stress (force exerted to the disc based on an air pressure created from rotation) that the substrate receives predictably becomes larger as the substrate spins with a higher speed. Based on a dynamics theory, flexion W of a disc which receives load P per unit area is denoted as follows: ##EQU1##
wherein: a represents the outer diameter of the disc; h represents the thickness of the substrate; E represents the Young's modulus of the disc material. Only gravity is exerted to the disc at a still state. The warp is indicated as, where the specific gravity of the disc material is represented by d, ##EQU2##
Herein, G represents the specific modulus of elasticity (=Young's modulus/specific gravity) of the disc material. Meanwhile, in the case that the gravity component can be neglected upon balancing centrifugal components in a rotational state of the disc, the force exerted to the disc can be deemed as air pressure based on the rotation. Such an air pressure is a function relating to the rotation speed of the disc, and it can be said generally as proportional to square of the speed. Accordingly, where the disc spins at a high speed the warp W can be represented as follows: ##EQU3##
According to this consequence, it turns out that a substrate material having a high Young's modulus is required to suppress the vibration in the substrate that is subject to high speed spinning. From a calculation done by the inventors, the specific modulus of elasticity of the substrate material is required to be at least 37 MNm/kg or higher if the substrate thickness is reduced to 0.43 mm from 0.635 mm in the case of 2.5 inch substrates and to 0.635 mm from 0.8 mm in the case of 3.5 inch substrates. If the rotation speed of the 3.5 inch high end substrate is made faster from current 7,200 rpm to future 10,000 rpm, the aluminum substrate having a Young's modulus of around 70 Gpa cannot correspond to it, and a new substrate material having a Young's modulus of at least 110 Gpa or higher is required. Because the substrate has not only a higher rigidity but also a higher impact resistance and strength as the substrate material has higher specific modulus of elasticity and higher Young's modulus, the market of the hard disc drive strongly seeks a glass material having a higher modulus of elasticity and a higher Young's modulus.
The chemically reinforced glass as disclosed above in Japanese Unexamined Patent Publication No. 1-239,036, however, has a Young's modulus of about 80 Gpa, and it is apparent that such a glass cannot response to strict demands on upcoming hard discs. The conventional glass for ion exchange reinforced substrate has alkali ions in a large amount introduced into the glass for ion exchange, and therefore, the reinforced glass has a low Young's modulus (90Gpa) as well as a low rigidity, so that the glass cannot correspond to substrates for 3.5 inch high end disc or thinner disc. A large amount of alkali component can be contained in a glass subjecting to a chemical reinforcement by the ion exchange. Therefore, if the glass is used for a long time under a high temperature and moisture environment, alkali ions may be deposited from pin holes in the magnetic film, thin portions of the magnetic film such as vicinities of the magnetic film, or exposed portions of the glass and may disadvantageously induce corrosions and deterioration of the magnetic film. During the manufacturing process for the magnetic recording medium, a prescribed thermal process can be used for improving characteristics such as coercive force of a magnetic layer after the magnetic layer is formed on the glass substrate. With such a conventional ion exchange reinforced glass, however, the glass transition temperature is at most around 500.degree. C., and since the glass lacks heat resistance, there also raises a problem that the glass cannot obtain a high coercive force.
The conventional crystallized glass as disclosed above in Japanese Patent Publication No. 2,516,553 has a little better property than the above chemically reinforced glass substrate in terms of the Young's modulus and the heat resistance. However, it has the surface roughness of 10 angstroms or higher, poor surface smoothness, and a limitation against lower flying of the magnetic head. Therefore, there raises a problem that the glass may not correspond to high density trends of the magnetic recording. Furthermore, the Young's modulus is at most about 90 to 100 Gpa, so that such a glass cannot be used for substrates for 3.5 inch high end disc or thinner disc.
The crystallized glass disclosed in Japanese Unexamined Patent Publication No. 7-291,660 has a Young's modulus of at most 100 to 130 Gpa, which is inadequate for use. Moreover, the glass has a limited surface smoothness in which the mean roughness of central line (Ra) is about only eight angstroms and lacks smoothness. In addition, the glass has a high liquid phase temperature of about 1400.degree. C., and therefore, the glass has a disadvantage that the glass is hardly subject to a high temperature melting and high temperature molding.